1. Field of the Invention
This invention relates to a method of diffusing an impurity in the formation of semiconductor materials. More particularly, it relates to a method of diffusing an impurity of high concentration into a semiconductor wafer at high efficiency, the method being especially suitable for the diffusion of a high concentration of antimony.
2. Description of the Prior Art
As is well known, the sealed tube process and the open tube process are usually employed for vapor-diffusing impurities into semiconductor wafers.
The sealed-tube diffusion process is a method in which semiconductor wafers and the source of an impurity to be diffused are contained in a quartz tube and are heated therein. The method can diffuse the impurity uniformly by holding the heating temperature constant. Since, however, a sealed tube is expensive and operations are difficult, the method is scarcely used in semiconductor processes except in cases where the impurity source is highly toxic, where an extremely high uniformity is required of a diffused layer, etc.
On the other hand, the open-tube diffusion process is a method in which semiconductor wafers are placed in a reaction tube opened at both ends and are heated while feeding into the tube a gas containing an impurity. When a substance having a sufficient vapor pressure at the diffusing temperature, for example, phosphorus or boron, is used as the impurity source, it is easy to establish the diffusion controlled state and the uniformity and sheet resistance (.rho..sub.s) of a diffused layer obtained are rarely problematic.
However, in a case where an impurity incapable of attaining a sufficiently high vapor pressure at the diffusing temperature, for example, antimony, is diffused, the following problems are liable to occur; the surface impurity concentration of a diffused layer does not become high, and the uniformity of the diffused layer is unsatisfactory.
In the case of diffusing antimony, the surface impurity concentration of the diffused layer is determined by the vapor pressure of antimony on the wafer surface to be formed with the diffused layer. Therefore, when antimony glass is formed on the surface of the wafer and antimony is diffused into the wafer by employing the antimony glass as the impurity source, an impurity diffusion can be carried out that is more stable than a method not using the antimony glass.
For this reason, in diffusing antimony, etc., the method which uses glasses containing antimony, etc., is relied on most extensively. When the contents of antimony, etc., in the glasses are fixed in advance, the quantities of antimony, etc., to be supplied into or arrive into the wafer surfaces are fixed, and hence, the surface impurity concentrations and diffusion depths of diffused layers to be produced by the diffusion treatments are substantially fixed.
Since, however, the method forming the antimony glass on the wafer surface and employing it as the source of the diffusion is extremely complicated in practical use, it is not favorable for the mass production of semiconductor devices, and a more convenient method has been desired.